The Mississippi River flooding, drought and extreme temperatures all could have harmed shrimp growth and survival last year. Temperature and salinity can affect shrimp metabolism and growth, in part by decreasing the worms and other smaller crustaceans that shrimp feed on in the estuaries as they grow to adulthood, according to Lawrence Rozas, with NOAA Fisheries Service’s Estuarine Habitats and Coastal Fisheries Center in Lafayette.

Chris Swarzenski, with the U.S. Geological Survey’s Louisiana Water Science Center in Baton Rouge, compiled data sets for The Times-Picayune on salinity and temperature variability in coastal Louisiana estuaries for 2010, 2011 and a 15-year historical average of the years 1997 through 2011. As expected, salinity levels varied among estuaries, especially due to differing influences of the Mississippi River flood and the drought.

Anecdotal data from shrimpers and state Wildlife and Fisheries officials point to areas east of the Mississippi River, which had lower salinity levels, having a lower shrimp catch than areas west of the Mississippi, which showed higher salinity.

While no published studies yet examine the direct relationship between shrimp and the Mississippi River flooding, drought and extreme temperatures last year, past studies have found that ideal salinity for shrimp growth ranges between 5 and 25 parts per thousand. Beyond that range, growth and mortality rates can worsen. Salinity levels are measured by grams of salt per kilogram of water, with the open ocean typically at about 35 parts per thousand.

The studies have shown that brown shrimp growth rates at salinities of 8 to 12 parts per thousand were significantly greater than growth rates at salinities between 2 and 4 parts per thousand, and more recent studies last year by Rozas and Thomas Minello, with the NOAA Fisheries Service’s Galveston Laboratory, have suggested that salinity may affect shrimp growth indirectly by decreasing their food source.

Swarzenski’s data sets show that some areas unaffected by flooding had more drought effects, such as Calcasieu Lake near Hackberry, which showed salinity hitting about 25 parts per thousand, nearly doubling the 15-year average.

During portions of the year, Caillou Bay southwest of Cocodrie also rose above 25 parts per thousand, much higher than long-term averages. Then, during the flooding, salinity there dipped below historic levels, according to the USGS data.

Because of the opening of the Davis Pond and Caernarvon diversions in 2010, salinity levels often showed even more extreme lows in 2010 than in 2011, although because of extensive shrimp fisheries closures in oil spill areas, it would be difficult to separate the potential effects on annual shrimp catch.

Only 38.9 million pounds of headless Louisiana shrimp were caught in 2010, compared with 57.2 million pounds in 2009 and 49.6 million pounds in 2011.

And in 2010, salinity levels in Barataria Bay near Grand Isle and Gardene near Pointe a la Hatche dropped to well below 5 parts per thousand around June and July.

In 2011, salinity at Barataria Bay was much lower than historic averages for much of the year, but did not dip as deep as the year before.

Reviewing the USGS data, Rozas, who has focused heavily on Gulf shrimp’s relationship to salinity and temperature, noted that while water temperature plotted for the different stations does not appear to have been affected much by the flood, the hotter-than-average summer or the drought of 2011 when compared with the 15-year average, the USGS salinity and temperature data is just at one point within an estuary. And there can be high spatial variability in temperature or salinity within an estuary, he said.

In addition, decreased oxygen levels, which occurred last year, can kill organisms in bottom sediment in the Gulf that are a source of food for shrimp, and can cause shrimp migrations from non-optimal temperature and salinity environments, according to Kevin Craig, a NOAA fisheries biologist.

And, in general, if shrimp grow more slowly, they spend more of their lifespan in that vulnerable juvenile state and predators can more easily attack them, leading to higher mortality